Nanostructured materials have opened the door to realizing devices with ultra-miniature sizes and ultra low electric consumptions, which conventional materials could not have achieved. Nanostructured materials, which can be defined as materials with crystallite sizes less than 100 nm in dimension, are typically synthesized by either “bottom-up” or “top-down” processes. The bottom-up process starts with atoms, ions or molecules as “building blocks” and assembles nanoscale clusters or bulk material from them. The “top-down” methods for processing of nanostructured materials involve starting with a bulk solid and obtaining a nanostructure by structural decomposition. One such approach involves the lithography/etching of bulk material analogous to the processes used in the semiconductor industry wherein devices are formed out of an electronic substrate by pattern formation (such as electron beam lithography) and pattern transfer processes (such as reactive ion etching) to make structures at the nanoscale.
Carbon nanotubes (CNTs) are expected to be adopted for many applications because of their superior electrical and mechanical characteristics. Moreover, their unique structures are also attractive for sensor applications. For example, in the case of using semi conducting CNTs as a sensor, it is possible to identify gases based on the selection of a donor or an acceptor by control of electron charity. In contrast, conventional gas sensors only detect the change of the electric resistance by gas absorption. However, as this new type of sensor uses only one CNT as a sensor probe for detection of gases, many difficulties remain in producing such a device.
For example, in order to produce the gas sensor described above, the CNT must be isolated from the carbon soot that was prepared, and it must be moved and set on a desirable point via a “manipulation” process. Today, the manipulations of CNTs are performed using hand-made nano-tweezers used in a transmitting electron microscope (TEM), and carried out using the “top-down” method in this special and limited environment. However, these operations are not adaptable to make uniform devices or to set a plurality of sensors on one chip.
On the other hand, a gas sensor containing an anode with a vertically aligned CNT array and a cathode has been reported in the prior art. The sensor works by applying a DC voltage to two electrodes, and flowing gas between those electrodes. Ionized gas produced by the voltage affects a breakdown voltage of the CNT array. By observing the differences in the breakdown voltage, the type of gas can be identified.
Vertically aligned CNT arrays are currently produced using the “bottom-up” method. In particular, the CNT arrays are generally made using a chemical vapor deposition (CVD) process with catalysts, namely the pyrolysis of compounds containing a carbon source and catalyst elements. Based on these CNT arrays, CNTs with proper alignment are produced easily, such that a plurality of sensors can be set on single chip.
Other sensors based on CNT arrays have been disclosed in the prior art. For example, U.S. Patent Application Publication No. 2004/0004485 discloses a sound sensor to detect sounds by observing a change in capacitance between two CNTs which face each other. Bias voltages are necessary to detect signals. Because the disclosed CNTs made on the electrodes have uniform lengths, the sensor is only useful for detecting a specific frequency. U.S. Pat. No. 6,737,939 discloses a radio frequency (RF) filter device which uses a CNT array on a substrate. The CNT array also contains CNTs that are uniform in length and cross section. By loading a bias voltage to the RF filter device, electrons on the CNT surface are increased. As a result, a quantum effect is caused which changes the lengths of the CNTs on the device, thereby changing a frequency for detection. U.S. Pat. No. 6,445,006 extends a CNT array disclosed by U.S. Pat. No. 5,872,422 for a Field Emission Display (FED) to detection applications used in micro-devices, such as Micro-Electro-Mechanical System (MEMS) based devices like an accelerometer or a flow meter. The disclosed detection principle is the physical contact originated between two CNTs electrically or the change of capacitance between them. U.S. Pat. No. 6,286,226 discloses a touch sensor using a CNT array to detect electronically a physical contact. However, a bias voltage is necessary to be loaded first.
Therefore, there remains a need for improved touch and auditory sensors.